1. Field of the Invention
The present invention relates to a method for reconstructing a planar tomogram of an examination subject from magnetic resonance signals in an inhomogeneous magnetic field with a magnetic resonance device that has a known inhomogeneous main magnetic field and/or at least one known non-linear gradient magnetic field.
2. Description of the Prior Art
Imaging by means of magnetic resonance techniques makes use of the frequency dependency of the magnetic resonance signal on the magnetic field strength for purposes of spatial resolution. Common methods for reconstructing tomograms presume a homogenous main magnetic field and strictly linear gradient magnetic fields. Inhomogeneities of the main magnetic field cause distortions or deformations in the frequency encoding direction. Given non-linearities of the gradient fields, the distortions are present not only in the tomographic image plane, but also perpendicular thereto, given slice excitations with a selection gradient. The distortions or deformations relate to the geometric position of the reconstructed spin density in the examination subject and likewise to the reconstructed image intensity.
Heretofore, distortions have been corrected in the image plane, exclusively. PCT Application WO 95/30908 teaches a method wherein a generalized Fresnel transformation is performed in the readout direction (GFT reconstruction). The GFT transformation takes into account a known location dependency of the main magnetic field in the readout direction.
The method described in German OS 195 40 837 uses two auxiliary data records which describe a shifting of the measured location relative to an actual location of a signal origin. From one of the auxiliary data records, a corrected auxiliary data record is created. In an image data record, a location correction then occurs in a first coordinate direction using the corrected auxiliary data record. A first intensity correction also occurs. Subsequently, a location correction in the second coordinate direction occurs, along with a second intensity correction. Alternatively, the location correction can occur by a Fresnel transformation of the raw data record including the corrected auxiliary data.
The article "Simulation of the influence of magnetic field inhomogeneity and distortion correction in MR imaging" (J. Weiss, L. Budinsky, Magnetic Resonance Imaging, 8, 1990: 483-489) teaches the correction of image distortions by a postprocessing of a conventionally acquired image. The information about magnetic field inhomogeneities which is required for this is obtained from the phase of separately registered spin-echo images. Deformations in the slice direction, i.e. slice curvatures, cannot be corrected with this method.
The slice curvature problem with respect to the main magnetic field non-homogeneity conventionally has been avoided by a 3D imaging wherein a spatial resolution occurs in the slice direction by means of an additional phase encoding. The phase encoding is relatively insensitive to main magnetic field inhomogeneities relating to deformations. The longer measuring time compared to multi-slice pickups and the resulting higher susceptibility to movement artefacts are disadvantageous, however. There are consequently fundamental restrictions in the application of certain techniques which are based on a rapid imaging, such as contrast agent examination or dynamic studies. Deformations relating to non-linear gradient fields heretofore have remained uncorrected.